The invention relates to a brake booster, and more particularly, to a brake booster which assures that an increased output can be obtained in response to a force of depression of a small magnitude which is applied to a brake pedal in the event a braking effort of an increased magnitude is required as in an emergency brake.
A brake booster which is arranged to permit an increased output to be obtained even in response to a brake depressing force of a small magnitude as in an emergency brake which requires a braking effort of an increased magnitude is known in the art, as exemplified by Japanese Patent No. 3,070,672 disclosing a brake booster in which in response to an input which exceeds a given value, an output gradient is increased subsequently to increase the output, or a Japanese Laid-Open Patent Application No. 35,474/1998 disclosing a brake booster in which an operation of a brake pedal is electrically detected, and the output is increased in response thereto.
However, drawbacks are pointed out with conventional brake boosters as mentioned above. Specifically, in the brake booster disclosed in ""672 patent, the arrangement is such that the output is increased when the input has exceeded the given value, and accordingly, the output cannot be immediately increased in response to a quick depression of the brake pedal.
On the other hand, in the brake booster disclosed in Application No. 35,474/1998, there is a need to provide a solenoid operated valve and a sensor which detects the degree of depression of the brake pedal in an integral manner with the booster, resulting in a complicated arrangement and an increased manufacturing cost of the brake booster.
In view of the foregoing, it is an object of the present invention to provide a brake booster which is simple in construction and which is capable of immediately increasing the output in response to a quick depression of a brake pedal.
Specifically, the present invention relates to a brake booster including a valve body slidably disposed within a shell, a power piston mounted on the valve body, a constant pressure chamber disposed forwardly of the power piston within the shell, a variable pressure chamber disposed rearwardly of the power piston within the shell, a valve mechanism disposed within the valve body for controlling a switching between the supply/discharge of a fluid to and from the variable pressure chamber, and an input shaft for operating the valve mechanism, the valve mechanism comprising a valve plunger connected to the input shaft and having an atmosphere valve seat, a vacuum valve seat formed within the valve body and a valve element which can be seated upon the atmosphere valve seat and the vacuum valve seat. According to the first aspect of the present invention, the brake booster further comprises a tubular member slidably disposed within the valve body and having the vacuum valve seat formed thereon, retainer means for causing the tubular member to be retracted relative to the valve body so that the vacuum valve seat is seated upon the valve element when the input shaft is advanced from its inoperative position through a given stroke relative to the valve body, and for retaining the tubular member at a position which is reached by retracting through a given stroke relative to the valve body, and release means for releasing the retention of the tubular member by the retainer means to allow the tubular member to return to its inoperative position when the input shaft returns to its inoperative position.
The second aspect of the present invention relates to a brake booster including a valve body slidably disposed within a shell, a power piston mounted on the valve body, a constant pressure chamber formed forwardly of and a variable pressure chamber formed rearwardly of the power piston within the shell, a valve mechanism disposed within the valve body for switching a supply/discharge of a fluid to or from the variable pressure chamber, and an input shaft for operating the valve mechanism, the valve mechanism comprising a valve plunger connected to the input shaft and having an atmosphere valve seat formed thereon, a first vacuum valve seat formed on the valve body and a valve element which can be seated upon the atmosphere valve seat and the first vacuum valve seat. According to the second aspect of the present invention, the brake booster further comprises a tubular member slidably disposed within the valve body and having a second vacuum valve seat juxtaposed to the first vacuum valve seat, retainer means for causing the tubular member to be retracted relative to the valve body to permit the second vacuum valve seat to be seated upon the valve element when the input shaft is advanced from its inoperative position through a given stroke relative to the valve body and for retaining the tubular member at a position which is reached by retracting through a given stroke relative to the valve body, and release means for releasing the retention of the tubular member by the retainer means to allow the tubular member to return to its inoperative position when the input shaft has returned to its inoperative position.
With the described arrangement, during an emergency brake operation in which a brake pedal is quickly depressed, the input shaft is advanced from its inoperative position through a given stroke or more relative to the valve body, whereby the tubular member is caused to retract relative to the valve body. Consequently, the vacuum valve seat (second vacuum valve seat) formed on the tubular member and the valve element which is seated thereon are caused to retract through a given stroke or more relative to the valve body and are retained in such positions by the retainer means. This allows an atmosphere valve which is formed by the combination of the atmosphere valve seat and the valve element to be opened to a greater opening than during a normal operation, allowing the atmosphere to be rapidly introduced into the variable pressure chamber to enable an output from the brake booster to increase rapidly to a greater value. In this manner, there is provided a brake booster which is simple in construction and which is capable of immediately increasing the output in response to a quick depression of a brake pedal.
Above and other objects, features and advantages of the invention will become apparent from the following description of the embodiments thereof with respect to the attached drawings.